A man found guilty of neglecting his infant son was put in custody during his trial because he threatened a doctor who gave evidence for the Crown.

While walking down the courtroom steps after the trial's first day on Monday, the man eyed a paediatrician who was part way through giving evidence for the Crown and said "thanks for ruining the last two years of our lives".

After the second day of the trial, Judge Tony Adeane heard from the man's lawyer, Scott Jefferson, who said his client admitted making the comment to Dr Philip Moore, but had not meant it as a threat.

"His frustration came to a head," Jefferson said.

Judge Adeane said the exchange was "plainly foolish" and revealed a "certain impulsiveness" displayed by the man in court.

He revoked the man's bail and put him in custody for the rest of the trial.

The man, 43, and his wife, 35, were yesterday found guilty of wilfully neglecting the boy between his birth on January 26, 2011, and March 25 that year.

The boy and his older sister have not been in the couple's care since April 2011.

The jury took less than two hours to reach their verdict after the three-day trial.

The couple, who have name suppression, were both remanded in custody to be sentenced in March, but Judge Adeane later allowed the woman to be released on bail.

They face a maximum sentence of two years' prison.

Fairfax Media can also reveal the man was issued a trespass notice by the Hawke's Bay District Health Board for threatening behaviour in Hawke's Bay Hospital in April 2011.

During the trial the jury heard the boy suffered multiple fractures while living with his parents in Lower Hutt.

His injuries were not discovered until early April, after the mother moved to Hastings and took him to a doctor for his immunisation shots.

When it was noticed he had a badly swollen thigh he was sent to hospital, where X-rays showed fractures to both legs, his pelvis and left big toe.

Moore spoke to the couple at hospital and a police investigation began.

Moore and a paediatric radiologist, Russell Metcalfe, told the jury the boy's injuries would have been "excruciating" and must have been evident to his parents.

The mother told the court the couple were not the baby's biological parents and that she falsified a birth certificate and lied to doctors and police.

Judge Adeane said he would be seeking further information before sentencing to see how blame could be apportioned.

The officer in charge of the case, Detective Toni Leppien, said police would investigate the incident involving Moore.